Microfiltration membranes are well known for use in removing particulates as small as 0.1 μm diameter and microorganisms as small as about 0.2 μm diameter from fluids such as water and air. These microporous membranes are made of organic polymers or sintered inorganic materials. In order to achieve these levels of filtration performance, microfiltration membrane filters such as those for use in pharmaceutical applications must pass costly, rigid quality control tests. Failed products must be discarded or downgraded for use in less demanding applications. Moreover, it is not unusual to observe bacteria growing through a microporous filter when water is stagnated in the filter for a period of time such as overnight.
Microfiltration filters such as pleated membranes and hollow fiber membrane (HFM) modules designed to filter microorganisms from drinking water are known. However, quality control of these types of products which are sold to consumer markets such as for use with drinking water are usually less stringent then those for pharmaceutical applications. Furthermore, the filtration performance of microfiltration filters intended for use with drinking water may be reduced by plugging caused by small particles and insoluble organic matter in the influent water which plug the filter.
Various prefiltration methods and materials have been used to protect microfiltration filters against undesirable plugging. Although these methods and materials are relatively expensive, it is known that the application of a pre-coat of fine particulate to an ordinary septum filter may significantly improve the efficiency and reliability of the filter as well as its useful life. The areal density of the pre-coat of fine particulate is typically more than 200 g/m2. It is also known to add bacteriastatic agents such as silver, copper, and zinc ion release media to the influent surfaces of microfiltration filters to prevent or reduce any bacteria retained in the filter from growing. However, these agents may not kill all of the bacteria and none of these agents have been employed to treat the filtered effluent water to kill remaining bacteria in the effluent.
Although the pre-coat prefiltration methods and materials of the prior art have been useful for improving filtration efficiency, filter life and reliability, these methods must build a layer of filter cake of particulate over all of the filtration surfaces. A continuing need therefore exists for low cost prefiltration methods to protect microfiltration filters and to improve their reliability to remove microorganisms such as those larger than 0.2 μm in diameter for use in applications such as filtration of drinking water.
A continuing need also exists for improving the ability of microfiltration filters to achieve increased removal of bacteria from water.
It also is known that when installing a carbon block or hollow fiber membrane module filter inside a filter casing, an end-cap adapter or an O-ring seal is used to make the connection and achieve a tight seal. The disadvantage of this known method is high cost and lack of flexibility to adjust the location of the hollow fiber membrane module in the filter casing as well as lack of reliability of the rubber O-ring seal. Moreover, even though double O-ring designs have been used in filters to reduce the likelihood of failure, the O-ring may be displaced during installation of the membrane module and lead to failure.
A need therefore exists for filters which overcome the disadvantages of the prior art.
Methods also are known in the art to stack pleated microfiltration filter modules to achieve flow rates greater than those of single pleated microfiltration filter modules. These methods, however, have not been applied to produce stacked hollow fiber membrane modules due to flow direction in the hollow fiber membrane modules. A need therefore also exists for a method for stacking hollow fiber membrane modules and filter cartridge devices which employ stacked hollow fiber membranes.